Let x be the number of one-striped fish and y the number of two-striped fish.
100 families each have 2 fish, so there are a total of 200 fish:
x + y = 200
x one-striped fish carry a total of x stripes, while y two-striped fish carry a total of 2y stripes. There are 273 stripes overall, so
x + 2y = 273
Solve for x and y. Subtracting the first equation from the second eliminates x :
(x + 2y) - (x + y) = 273 - 200 ==> y = 73
Then
x + 73 = 200 ==> x = 127